The distribution of heme-containing respiratory pigments capable of reversibly binding oxygen is well documented throughout the animal kingdom. Work in our laboratory has shown that at least two protozoan speices, Paramecium tetraurelia and Paramecium multimicronucleatum contain such respiratory pigments. This pigment in P. tetraurelia is present in five electrophoretically different forms which appear to be identical in molecular weight and amino acid composition and by spectral analysis. The present project is aimed at determining as much of the primary sequence of the five varient forms to allow a comparison of their primary sequence with each other and those other myoglobin species that have been sequenced to date. To data only one form, Mb-4, has been isolated in sufficient amounts and purity to allow partial sequence work to begin.